The invention relates generally to fan drive systems and more particularly to hydraulic and wet friction-type clutches for fan drive systems. There are various types of friction coupling devices and fluid coupling devices used to drive various devices or systems, such as radiator cooling fans for internal combustion engines. These friction clutch devices generally include dry friction clutch assemblies, viscous drive clutch assemblies, and wet friction clutch assemblies. Dry friction clutch assemblies have only two stages of operation: fully engaged or fully disengaged. Dry friction clutch assemblies also generally have low thermal capacity since they typically do not incorporate fluid flow cooling mechanisms. Viscous drive clutch assemblies have the ability to engage at higher engine speeds and can have varying degrees of engagement. Viscous drives are never fully engaged for internal viscous sheer purposes.
Viscous drives slip to some degree at all times, making them incapable of turning at fully engaged peak operating speeds or at higher speeds than originally designed. Viscous drives are further limited in that the more engine cooling that is needed, the larger and more costly the viscous drive and cooling fan that are required.
Wet friction clutches are popular particularly for their use in situations involving severe service where the fan drives are in constant service and carry a constant load. Wet friction systems have the advantage of a friction clutch assembly as well as the ability to provide increased engine cooling.
Wet friction fan drive assemblies are shown, for example, in U.S. Pat. Nos. 7,047,911, 7,249,664 and 7,178,656. These systems utilize hydraulically controlled fan drives with certain methods of engagement. The hydraulic systems include a housing assembly containing a hydraulic fluid and an engaging circuit. The engaging circuit includes a pitot tube coupled within the housing assembly that receives at least a portion of the hydraulic fluid. An energizing circuit engages the housing assembly to a fan shaft in response to supplying the hydraulic fluid from the pitot tube.
Although these wet friction clutch systems described above provide improved fan drive control systems and assemblies, particularly as to the engagement pressure and control, as well as the removal of internally generated heat, it is an object of the present invention to provide a further improved assembly and system.